A speaker grille is designed so that it has a sufficient mechanical strength needed for protecting a speaker from damage, and at the same time it has to provide a speaker with an opening that is large enough not to ill-affect the quality of reproduced sounds. For those speakers whose diaphragm areas are small, among other speaker types, new structures have been proposed for the speaker grilles in order to minimize the area of the diaphragm region that is blocked, while securing sufficient mechanical strength. Japanese Patent Unexamined Publication No. 2003-37883 discloses a conventional speaker grille 91 as shown in FIG. 27; which speaker grille is disposed in front of speaker 92, and provided with sound openings 93 whose open rates are uneven.
Speaker grilles require quite a high open rate to avoid retarding sound vibration of the speakers. The main sound openings are disposed in an area corresponding to the central region of a diaphragm, where the sound pressure level is high. Describing more practically, the total area of sound openings has to be at least 60%, if the characteristics are to be kept within approximately −6 dB for the sounds of 5 kHz and higher. The smaller the speaker size is, the higher the open rate that is required for the speaker grille.
FIG. 28 is a sound characteristics chart of a speaker combined with a speaker grille whose open rate is as low as 28%. In the chart, the solid line represents sound characteristics 95 with no speaker grille attached, while the dotted line represents sound characteristics 96 with the speaker grille attached. The sound pressure level is lower in the vicinity of 5 kHz and of 10 kHz with sound characteristics 96 shown in dotted line, as compared with characteristics 95. On the other hand, sound characteristics 96 exhibit a higher sound pressure level in the vicinity of 3 kHz; which is due to resonance. Thus, design in the sound openings of a speaker grille significantly influences the sound characteristics of a speaker combined with the speaker grille.
The intrinsic task of a speaker grille is to protect a speaker from damage, so it has to have a certain mechanical strength. Therefore, there is a limitation to making the open rate of the sound openings larger. The speaker grille as shown in FIG. 27 secures a certain open rate by providing rectangular sound openings in the area opposing the diaphragm edge, in addition to the area opposing the central part of the diaphragm. This configuration, however, requires a certain thickness for ensuring a required mechanical strength. This means that it is not suitable for use with the small-size speakers, among other types of speakers.